FIELD OF THE INVENTION
The invention relates to the field of catalytic hydrocarbon gas reformers, and more particularly to an oxygen gauge which assists in monitoring ratios of H.sub.2 O (steam) to gaseous hydrocarbon, or in other words oxygen to carbon (O:C), in a hydrocarbon feed gas stream entering or leaving a hydrocarbon gas reformer of an electrochemical fuel cell generator, such as a high temperature, solid oxide electrolyte, fuel cell generator.
Natural gas, for example comprising methane (CH.sub.4), ethane (C.sub.2 H.sub.6), propane (C.sub.3 H.sub.8), butane (C.sub.4 H.sub.10) and nitrogen (N.sub.2), vaporized petroleum fractions such as naphtha, and also alcohols such as ethyl alcohol (C.sub.2 H.sub.5 OH), are appropriate fuels for an electrochemical generator apparatus, such as a high temperature, solid oxide electrolyte fuel cell generator, for generating electrical power. However, the direct use of hydrocarbon fuels can result in carbon deposition within the generator apparatus partly due to hydrocarbon cracking. Carbon deposition tends to reduce the life of the components of the generator apparatus, such as the fuel cells, insulation boards, and reforming catalysts, which in turn reduces efficiency of the generator. To reduce carbon deposition, it is known to reform the natural gas or other hydrocarbon fuel gases into simpler molecules, namely into carbon monoxide (CO) and hydrogen (h.sub.2), through the use of a reforming catalyst and added water vapor (steam) and/or carbon dioxide (CO.sub.2) prior to introduction of the fuel gases into the fuel cell generator. The use of the CO and H.sub.2 produced from reforming effectively provides the oxidizable fuel for the electrochemical fuel cells, and carbon deposition is thereby reduced or eliminated.
The presence of water vapor and/or carbon dioxide and a reforming catalyst, such as finely divided Ni or finely divided Ni plus MgO mixtures, allows the conversion of gaseous hydrocarbon fuels, typically natural gas comprising in most part methane (CH.sub.4), to CO and H.sub.2 by the endothermic reforming reaction which is typically performed at about 600.degree. C. to 900.degree. C. The reforming reactions for methane are represented by Equations (1) and (2). EQU CH.sub.4 +H.sub.2 O(g).revreaction.3H.sub.2 +CO (1) EQU CH.sub.4 +CO.sub.2 .revreaction.2H.sub.2 +2CO (2)
The reformed fuel is then combined, for example, in a high temperature, solid oxide fuel cell generator, with an oxidant, such as air or O.sub.2, to produce electrical energy and heat. If the reforming reaction has insufficient oxygen in the form of H.sub.2 O (steam) and CO.sub.2, undesirable carbon deposition will occur partly due to hydrocarbon cracking. A typical natural gas fueled, high temperature, solid oxide electrolyte fuel cell generator should operate with atomic ratios of O:C in the range of about 2.0:1 to 2.5:1 prior to reforming, to avoid carbon deposition.
Systems which reform natural gas or other hydrocarbon gases could be made more effective by controlling and maintaining the O:C ratio at a value sufficient to prevent the deposition of carbon in the system. However, a useful device to monitor the O:C ratio at the elevated temperatures of the hydrocarbon reformer, especially in an internal reformer of a high temperature, solid oxide electrolyte fuel cell generator, has not been taught. It would be desirable to provide a useful device to assist in monitoring O:C ratios of a reformable fuel gas mixture prior to reforming.
An electrochemical cell can be used as an oxygen sensor. An oxygen sensor based on a solid oxide electrolyte electrochemical cell is taught, for example, in U.S. Pat. No. Res. 28,792 (Ruka, et al.). At elevated temperatures, a potential difference is developed between the electrodes of the electrochemical cell, due to differences in oxygen partial pressures between a sample and a reference atmosphere across a solid electrolyte disposed between two electrodes, i.e., a cathode and an anode. This potential difference at a known temperature and reference oxygen partial pressure is coupled to an external electrical circuit which relates this potential difference to the oxygen partial pressure of the sample atmosphere contacting the electrochemical cell functioning as an oxygen sensor. The oxygen sensor thereby encodes oxygen partial pressure in a manner that can be used for monitoring and control purposes.
The pertinent electrochemical reaction at the cathode of the oxygen sensor where air is typically used as the reference gas is represented by Equation (3). EQU O.sub.2 +4e.sup.- .revreaction.2O.sup.2- ( 3)
The electrochemical reaction at the anode of the oxygen sensor where the oxygen partial pressure is typically lower is represented by Equation (4). EQU 2O.sup.2- .revreaction.O.sub.2 +4e.sup.- ( 4)
For calibration, in Ruka, et al., one of the electrodes of the electrochemical cell is used as a reference electrode over which flows an atmosphere with known oxygen partial pressure and total overall pressure. An oxygen containing atmosphere to be measured of known total pressure is exposed to the other electrode which can be termed the measurement or sensor electrode. The measured potential difference between the electrodes is used to calculate the unknown partial pressure of the oxygen at the sensor electrode and, thus, the oxygen concentration of the oxygen containing sample atmosphere.
Other gas sensor designs are taught in U.S. Pat. Nos. 4,377,460 (Hirayama, et al.), 4,391,690 (Lin, et al.), and 4,902,401 (Lin, et al.). These gas sensors are similarly solid electrolyte electrochemical cells which sense the equilibrium of a gas species of interest and generate an electromotive force (EMF) signal that represents the difference in partial pressure of the gas species across the ion conductive solid electrolyte sensor disposed between two electron conductive electrodes. The monitored gas stream contacts a sensor electrode while the opposite electrode serves as a reference electrode which is contacted with a reference gas stream. Conventional solid electrolyte compositions require operating temperatures typically between 200.degree. C. and 1000.degree. C. to exhibit the desired and almost exclusive ion conductivity of the solid electrolyte to generate a suitable EMF signal.
The EMF signals generated by an oxygen gas sensor are developed in accordance with the well-known Nernst equation represented by Equation (5). ##EQU1## where R=Universal Gas Constant; T=Absolute Temperature (.degree.K); F=Faraday Constant; P=Partial Pressure of Reference Gas; P.sup.1 =Partial Pressure of Monitored Gas; and, m=Number of Electrons Transferred=4 electrons per 20.sup.2- for the O.sub.2 cell.
From this equation and with knowledge of the oxygen partial pressure on the reference side and temperature, a direct measurement of the O.sub.2 gas partial pressure in the monitored gas environment on the measurement side can be made by measuring the EMF of the sensor cell or cells. The EMF can be used to drive a meter or a control circuit, or the level can be digitized or otherwise encoded as needed.
High temperature, solid oxide electrolyte fuel cells and multi-cell generators and configurations thereof designed for converting chemical energy into direct current electrical energy, typically in a temperature range of from 600.degree. C. to 1200.degree. C., are well known, and taught, for example in U.S. Pat. Nos. 4,395,468 (Isenberg) and U.S. Pat. No. 4,490,444 (Isenberg). There, a previously reformed hydrocarbon fuel converted to either H.sub.2 or CO is fed into the generator apparatus at one end and flows parallel to the exterior fuel electrode surfaces of the fuel cells. Spent fuel is combusted with spent oxidant to release heat and then exits the generator apparatus.
Other high temperature, solid oxide electrolyte fuel cells are known where spent fuel containing H.sub.2 O and CO.sub.2 is recirculated and mixed with a fresh gaseous hydrocarbon fuel at an ejector, nozzle or the like in the interior of the generator apparatus. The gaseous mixture is then fed through an internal catalytic hydrocarbon reformer to produce a reformed fuel gas, such as H.sub.2 and CO, which is fed to contact the exterior fuel electrode surfaces of the fuel cells in the generator apparatus, as taught in U.S. Pat. No. 4,729,931 (Grimble). Other high temperature, solid oxide electrolyte fuel cell generator apparatus having internal catalytic hydrocarbon reformers are taught in U.S. Pat. No. 4,729,931 (Grimble); U.S. Pat. No. 4,983,471 (Reichner, et al.); U.S. Pat. No. 5,047,299 (Shockling); U.S. Pat. No. 5,082,751 (Reichner) U.S. Pat. No. 5,143,800 (George, et al.); and, U.S. Pat. No. 5,169,730 (Reichner, et al.).
Use of recirculated spent fuel gases produced from electrochemical operations in an electrochemical generator apparatus, such as a solid oxide fuel cell generator, to provide the oxygen species required for hydrocarbon reforming, namely gaseous H.sub.2 O and/or CO.sub.2, for combination with gaseous fresh hydrocarbon feed fuel to produce a reformable gas mixture, e.g., by means of an ejector or aspirator powered by the inlet fresh hydrocarbon feed fuel pressure, potentially has several problems. The reformable gas mixture, however, must be closely regulated to provide proper generator operations and eliminate carbon deposition. To address certain problems, the O:C ratios of the feed gas entering a reforming chamber should be optimally monitored and controlled to maintain desired levels. In this manner, it is possible to reduce or eliminate unwanted carbon deposition that may occur when the ratio is improper. In practical applications, the composition of the fresh hydrocarbon feed fuel gas typically may vary, requiring control of the amount passing through the ejector, and also control of the amount of water vapor and/or carbon dioxide laden spent fuel gas. For example, too great a draw of spent fuel gas at the ejector, will reduce the Nernst potential in a significant portion of the generating chamber of the apparatus and will result in lower operating voltage or poorer utilization of fuel than would otherwise be possible.
However, it is not currently possible to effectively monitor and control O:C ratios of a feed gas entering a reforming chamber of an electrochemical generator with a conventional high temperature electrochemical cell functioning as an oxygen gas sensor, since system noise, e.g., temperature variations and sample gas concentration variations, destroy any meaningful EMF readings. Therefore, it would be advantageous to monitor the reformable gas mixture entering or exiting the reforming chamber by a novel and a substantially noise-free gas sensor to facilitate optimal hydrocarbon reforming, and prevent carbon deposition and insure proper recirculation rate of spent fuel gas.
What is needed is a gas sensor that can effectively monitor, preferably on-line, the O:C ratios of the inlet or outlet fuel gas streams of the reformer such as a reformer of a high temperature, solid oxide electrolyte fuel cell generator. Such an oxygen gas sensor coupled to the appropriate control equipment would prevent carbon deposition and prevent reduction in electrochemical generator efficiency.